Back problems can often be attributed to pathological changes in the area of the spinal column. One of the main causes lies in the interaction between two adjacent vertebrae and their connection. This in particular concerns intervertebral disks and facet joints. The connection between vertebrae may become damaged because of disease or overloading, either through injury or wear. This can lead to restricted movement, pain, or even loss of mobility. In the past, therapy was focussed on treating the intervertebral disks. Thus, various artificial joints were proposed which are implanted in place of a defective natural intervertebral disk. In this way, defects on the facet joints could also be indirectly compensated, and their consequences alleviated. However, it has been shown that this is not sufficient and that, in many cases, targeted therapy of the facet joints is also needed.
Various approaches have been attempted for treatment in the area of the facet joints. In the simplest case, treatment is restricted to reducing the pain caused by the facet joint, in particular by injection of analgesics in the area of the affected facet joint. A further possibility is that of removing the facet joint. This of course eliminates one of the causes of pain, but at the price of a significant change to the biomechanics of the spinal column. The absence of facet joints reduces the stability of the spinal column to a great extent and thus leads to greater loading of the other areas of the spinal column, which often leads to defects in the area of adjacent connection between vertebrae. The same applies to the reverse approach, namely that of immobilizing the facet joint by a fixed connection. Here too, the biomechanics are negatively affected. The mobility of the patient deteriorates, and there is greater loading in the area of the adjacent vertebrae.
It has also become known to provide the facet joints with a protective cap in the area of their articular surface. To create space for the cap, a considerable amount of material has to be removed from the natural bearing surface. This removal of tissue often leads to an interruption of the natural supply of surrounding healthy tissue, with the consequence of necrotic changes.
To avoid said disadvantages of these traditional treatment methods, various prostheses for facet joints have been developed.
A complicated intervertebral disk prosthesis, with additional facet joint prostheses that can be attached thereon, is known from WO-A-2004/098465. The prosthesis is thus suitable for simultaneous treatment of the intervertebral disk and of the facet joints. As a result of this combination, the prosthesis has a complicated structure and has relatively large dimensions. This necessitates a complex operation with an access route of large dimension. Treatment of the facet joints in the area of the cervical spine is difficult with this prosthesis.
A more compact prosthesis for a facet joint is known from WO-A-2005/037149. It comprises an elastic element with a disk-shaped thickening which is placed between the bearing surfaces of the facet joint. The prosthesis is secured by fastening elements that are introduced into through-holes in the corresponding articular processes. Since these through-holes have to be flush with one another, their creation is very difficult and requires an unimpeded access. The prosthesis further creates a flexible mechanical tensile connection of the facet joint, of a type that was not originally present.
Another special facet joint prosthesis is known from WO-A-2004/103227. It consists of two components which each have a bearing piece and a retaining piece for anchoring on the adjacent vertebrae. An upper component is anchored in the lamina of the vertebra by means of a screw. The screw has a relatively long shaft, which leads to unfavorable lever ratios and thus to a loading of the lamina. To avoid buckling of the screw, it has to have relatively large dimensions. The prosthesis comprises many protruding parts, which poses a danger of irritation of surrounding tissue. In addition, the fastening of the upper bearing component on the lamina instead of on the corresponding articular process changes the biomechanics. A similar facet joint prosthesis with a translaminar fastening screw is known from US-A-2005/0049705. The fastening screw lies with its head on the lamina and extends through a channel created in the vertebra to the lower facet joint surface, where a bearing piece is arranged and fixed by means of the fastening screw. The bearing piece interacts with a mating piece of the facet joint, which mating piece is arranged on the adjacent vertebra and is secured on the vertebra by means of a simple spike. A disadvantage of this prosthesis is that creating the translaminar channel is quite complicated, and a pass-fit joining of bearing piece and screw is difficult, since it depends on the particular angle of the facet joint surface. Considerable maneuvering difficulties arise particularly in the cervical area of the spinal column.
US-A-2004/0049272 discloses a facet joint prosthesis which comprises a bearing piece with a retaining rod rigidly mounted thereon and protruding into the vertebra. The bearing piece has a cup-shaped curvature and comprises a pressure plate for resting on the vertebra, and a bearing shell which interacts with a felt-type mating piece on the corresponding opposite face of the facet joint. The retaining rod can be connected permanently or releasably to the bearing piece. However, this prosthesis too is difficult to maneuver, especially in confined spatial conditions, such as those in the cervical spine area, if the retaining rod and mating piece have to be joined together in situ. If they are pre-assembled, then there is the difficulty of introducing them under the confined spatial conditions. For secure fixing, the retaining rod has to protrude far into the area of the vertebral body, which can lead to irritation there.
U.S. Pat. No. 177,240 discloses a facet joint prosthesis which comprises a bearing piece with a guide rod that can be screwed thereon. The bearing piece is designed, on its underside, as a pressure plate for resting on the vertebra, and, on its opposite side, as a bearing surface for interaction with a mating articular surface. At its opposite end, the guide rod has a mating element which can be provided with a screwdriver receiver. The guide rod is secured on the bearing piece by means of a screw connection. Under confined conditions, especially in the area of the cervical spine, creating a corresponding through-bore at an exact position, and connecting the bearing piece to the guide rod, can cause considerable difficulties.